A Passive Optical Network (PON) system is an optical transmission system to connect an Optical Line Terminal (OLT) at a central office to a plurality of Optical Network Units (ONUs) at end user's sides through passive optical elements which consist of optical fibers and optical splitters (See, for instance, Japanese Laid-Open patent application JP2005-175599).
A PON system is now widely used not only as Internet access lines but also as a system to provide broadcasts, data communications, and data.
When a PON system is used for broadcasting and/or telephone services, its functionality is facilitated by avoiding disconnection due to the failure of network equipment. Replacement of an OLT causes temporary interruption of service to all users under the OLT.